There have been previously known data-card readers which are suited for reading both punched cards and cards marked by writing. These devices have a system of one or several light sources arranged in the card guiding device opposite the card face, for emitting light in the direction of the data-card. Each light source cooperates with a detector for the light reflected by the card. The wall of the card guiding device facing the reverse side of the card is black so that to someone regarding the face of the card both the holes as well as the writing marks appear as black marks in the light-colored card material surrounding them. While the card material reflects a more or less uniform quantity of the light emitted by the light sources, holes and marks practically do not reflect any light at all. Accordingly, a medium signal level at the detector output signals "no databit", while a low signal level signals "databit".
The disadvantage of the known detector device must be seen in the fact that this device does not allow differentiation between punched holes and writing marks. When reading cards exhibiting only punched marks, there also exists the risk that impurities or stains on the card material may be read as databits.